The binder of this invention is a hybrid silicate binder for application to investment foundry molding processes. Investment foundries make use of three types of silicate binders. These are the alkali silicates (such as sodium silicate), organic ethyl silicates and colloidal silica sols. Each binder has its own advantages and one is not used universally over another.
Organic ethyl silicate, prehydrolyzed, is perhaps the most versatile of the silicates. Ethyl silicate has high refractoriness and can be chemically hardened with a given set time or hardened rapidly by air drying. Prehydrolyzed ethyl silicate produced in an alcohol base has the disadvantages of low bond strength, limited stability in refractory slurries and requires in-use explosion precautions.
Colloidal silica sol has advantages of high bond strength, good slurry stability and is aqueous based, thus eliminating fire hazards. The limitations of colloidal silica are that it cannot be chemically gelled and that its aqueous base produces slow air hardening.
Sodium silicate can be chemically hardened, but has the disadvantages of low refractoriness.
This invention is a novel approach to a binder which is a true synergistic reaction of colloidal silica sol and organic ethyl silicate providing a binder with the advantages of both silicates and has long shelf life and proven acceptance by investment casting foundries. Sodium silicate additions to the admixture were unsuccessful because of lowering binder refractoriness, shelf life and increasing cost. Also, glycol ether gels sodium silicate. Thus, sodium silicate is not usable for this invention. Polysilicates, described as silicates with very small particle size and low sodium to silica ratio may be substituted for colloidal silica.
Early attempts to achieve a universal binder were based on mixtures of ethyl silicate, colloidal silica sol and/or sodium silicate. A variety of patents attest to this approach. The Teicher U.S. Pat. No. 3,576,652 and the Emblem disclosure in U.S. Pat. No. 2,842,445 teach binder formulation with low colloidal silica sol volumes up to about 15% and high organic ethyl silicate volumes from 35% to 68%. The silica ratios in Teicher (Examples IA, IB, IC, and III) range from 2.03 to 14.5, while in Emblem (Examples 2, 3 and 7) they range from 5.8 to 13.6. Fujita disclosed, in U.S. Pat. No. 3,682,668, the addition of sodium silicate with ethyl silicate, however, high volumes of organic ethyl silicate were shown up to 80%. His silica ratio (Example III) is 1.87. These three patents teach binders of alcoholic base, which is a major limitation in binder properties and safety. All of these patents have failed to become accepted on a commercial basis for the following reasons:
1. The binders did not have long shelf life exceeding nine months because binders are made in alcoholic solutions.
2. Binder properties achieved were a compromise between the two binders and did not provide improved advantages to warrant production foundry use.
All three of these patents disclose binders having stability which is low in comparison to the binders made according to the present invention. If the examples were varied to approach the silica ratio here required, the binders would quickly gel because of the processes used. Such an approach would require an increase in silica sol which would lower stability. Thus my method must be used with my formula for successful results.